Add list of length and weight units

src/MDUnitsSet.php

@@ -0,0 +1,16 @@
+<?PHP
+/**
+ * Contains a class for controlling the list of available units.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+
+/**
+ * Class containing available units for weights, lengths etc.
+ */
+class MDUnitsSet {
+
+    const UNITS_LENGTH = ['', 'm', 'dm', 'cm', 'mm'];
+    const UNITS_WEIGHT = ['', 't', 'kg', 'g'];
+
+}